[SOLVED] Difference between DLLs ended with d or without d

I would like to know the difference between DLLs ended with d or without d (i.e. QtCore5.dll and QtCore5d.dll). I am developing and application which suddenly has changed it dependencies to the d version of DLLs and I don`t know the reason.

Just a little addendum: The "Release" versions are built with various compiler optimizations enabled and without debug symbols, which makes the code run (much) faster but makes debugging as well as reverse engineering more difficult. On the other hand, the "Debug" versions have most compiler optimizations disabled and they include debug symbols, so you can (for example) get meaningful stacktraces in your debugger. You want to make sure to onlydeploy your software with the "Release" versions ;-)